Contract cancelation

I had a 24moths cellphone contract with mtn over 2 years ago. Last year December my contract with them ended, meaning I completed paying for my cellphone. To my surprise in January mtn debited from my account, I did not understand why, I gave them a call and they explained that the debit is for the monthly Sim benefits, which is never stated when signing the contract by the way. I let it slid and humbly asked them to cancel that contract since I no longer needed those benefits the phone came with. They agreed and even gave me the reference no. To date the contract has not been canceled and they still debit every month. When I call they either tell me that their system is out of service or they hang up on me. I'm exhausted all methods I could use to reach to them, i visited their stores, sent them emails and called countless times. I've been their loyal customer for almost two decades now, I don't know why such treatment. It's like when you sign a contract with them you can't get off it. Please help.